In recent years, hybrid electric vehicles of various types, such as a series type and a parallel type, have been put into practical use. In such hybrid electric vehicles, electrical power supplied from a travelling battery causes an electric motor to act as a motor to move the vehicle, while the electric motor is caused to act as a generator using SOC (state of charge), which indicates the remaining capacity of a battery, as an indicator, and the battery is charged with power generated by the electric motor so that SOC can be maintained in a predetermined range. Thus, the travelling performance of a vehicle using an electric motor is greatly influenced by the charge-discharge performance of a battery, and various measures have been taken so that the battery can deliver excellent charge-discharge performance.
One of major factors that cause degradation in the charge-discharge performance of a battery is temperature decrease. For this reason, to realize excellent charge-discharge performance, it is especially effective to actively warm up a battery when the temperature of the battery decreases in a cold climate area or the like.
From this standpoint, various measures have been proposed so as to warm up a battery (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2003-272712). Japanese Laid-Open Patent Publication (Kokai) No. 2003-272712 describes a series hybrid electric vehicle powered by an electric motor acting as a motor using electricity from a generator driven by an engine, and in which when the temperature of a travelling battery decreases, the battery is charged and discharged repeatedly in short cycles within a predetermined SOC range so as to increase the temperature of the battery.